This invention relates to a novel method for determining the average width of photoluminescent stripes in a localized area of a luminescent viewing screen. The novel method is particularly useful for measuring the luminescent stripes in a shadow-mask color-television picture tube of the type having a striped viewing screen.
A shadow-mask picture tube comprises an evacuated envelope having therein a luminescent viewing screen comprising red-emitting, green-emitting and blue-emitting screen elements; electron-beam-producing means for exciting the screen to luminescence; and a shadow mask between the screen and the electron-beam-producing means. The mask is an apertured metal sheet that is precisely shaped and spaced adjacent the screen so that the mask apertures therein are systematically related to the screen elements. In one variety of shadow-mask picture tubes, the screen elements are parallel luminescent stripes and the apertures are arranged in columns that are parallel to the length of the stripes, and the apertures are elongated in the same direction. This type of screen and methods for making it are described in the prior art; for example, in U.S. Pat. No. 4,049,451 issued Sept. 22, 1977 to H. B. Law. The stripes are both cathodoluminescent (excitable by electrons) and photoluminescent (excitable by ultraviolet light).
In most prior methods, the screen of a tube is made photographically, using its shadow mask as a photographic master, on the inside surface of the viewing window of the faceplate panel of the tube. Subsequently, the panel with the screen and mask therein is assembled with other parts into a picture tube. After the screen is made, but before the panel is assembled with other parts, it may be desirable to determine the average widths of the luminescent stripes of the screen. This determination may be used for process control or for quality-assurance purposes.
A direct measurement using a microscope and reticule is too slow and too expensive for routine mass-production procedures. Furthermore, the accuracy of such a measurement is highly dependent on the skill of the microscope operator. A desirable determination is one that is fast, low in cost, accurate and not dependent upon human skills. The novel method satisfies these requirements.